The present invention relates to electrical device of motor bicycles, snow mobiles, or other industrial machinery which are sealed with resins so that they may be satisfactorily used under severe environmental conditions such as vibrations and cycling change in temperature over a very wide range (hereinafter referred to as cycling temperature change).
Hitherto, in order to protect the electrical devices from the severe environments, especially from vibrations, moisture and vapor, and cycling temperature change, they are sealed or enclosed with highly flexible resins such as epoxy resins, urethane resins, and so on. However, when a flexible resin is used in order to protect the electrical devices at low temperatures, the resistance to heat is not satisfactory. On the other hand, when a resin which is satisfactorily resistant to heat is used, its resistance to low temperature is not satisfactory. That is, the resistance to heat and the resistance to low temperature are incompatible with each other. In order to overcome this problem, a mixture consisting of a flexible resin and an insulating material has been used to seal or enclose electrical devices so that the coefficient of thermal expansion may be minimized. However, when an insulating material is mixed into a resin, the viscosity of the mixture is increased so that the electrical device cannot be completely set and sealed with the mixture. Thus, the use of such mixture cannot attain the satisfactory sealing effects of the electrical devices in practice.
The present invention was therefore made in order to solve the above and other problems encountered in the prior art techniques. One of the objects of the present invention is to provide resin-sealed electrical devices which may satisfactorily function even when they are subjected to severe cycling temperature change, vibrations and high humidity. In the present invention, an electrical device is not sealed with a mixture consisting of a resin and an insulating material, but is sealed by being embedded in one or more layers of the insulatng material among the particles of which sinks into a resin of low-viscosity. However, the grain size of the insulating compounds to be used must not be too coarse or too fine. When the grain size is too fine, it becomes very difficult to impregnate a resin from the top surface of an insulating material layer and as a result the impregnation time becomes inevitably longer. On the other hand, if the grain size is too coarse, the impregnated resin does not come short of the electrical elements constituting an electrical component. Therefore, when the electrical device is subjected to cycling temperature change cracks occur. It is of course easy to impregnate a resin into a layer of an insulating material of a coarse grain size. However, the grain size of the insulating material must be determined so that the desired electrical characteristics and performance of the resin-sealed electrical devices may be maintained.
Briefly stated, the present invention provides a resin sealed electrical device comprising an electrical element, a case for housing said electrical device therein, a layer of an insulating compound such as finely divided quartz (SiO.sub.2), magnesium oxide (MgO), lead oxide (PbO), borosilicate soda glass or rubber so that said electrical component may be embedded in said layer, and a cover layer consisting of a resin of low viscosity formed over said first mentioned layer, said resin of low viscosity being impregnated into said first mentioned layer.
According to one aspect of the present invention, an electrical component is embedded into a layer of a finely divided insulating material charged into a case, and a resin of low viscosity is poured into the case so that it may be impregnated into the insulating material layer and may form a cover layer over the free surface of the insulating material layer. Therefore, the resistance to moisture and vapor of the electrical device may be considerably improved, so that the deterioration of the electrical characteristics may be well prevented. Furthermore, the commercial value of the electrical device may be remarkably increased. Moreover, the performance of the electrical device sealed with resin in accordance with the present invention is very satisfactory under very severe environmental conditions; no crack is started; and the desired electrical characteristics may be maintained. According to another aspect of the present invention, the particles of different sizes of an insulating component are used so that a plurality of insulating material layers of different grain size ranges may be formed. Therefore, the impregnation of a resin of low viscosity is readily accomplished within a short time, so that the fabrication efficienty of the electrical device may be much improved.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of some preferred embodiments and examples thereof in conjunction with the accompanying drawings.